Thermal imaging or thermography is a recording process wherein images are generated by the use of imagewise modulated thermal energy.
In thermography three approaches are known:
1. Thermal dye transfer printing wherein a visible image pattern is formed by transfer of a coloured species from an image-wise heated donor element onto a receptor element;
2. Image-wise transfer of an ingredient necessary for the chemical or physical process bringing about changes in colour or optical density to a receptor element; and
3. Direct thermal formation of a visible image pattern by image-wise heating of a recording material containing matter that by chemical or physical process changes colour or optical density.
A survey of thermographic processes is given e.g. in the book "Unconventional Imaging Processes" by E. Brinckman, G. Delzenne, A. Poot and J. Willems, The Focal Press--London and New York (1978), Chapter 4 under the heading "4.10 Thermography".
Thermal dye transfer printing is a recording method wherein a dye-donor element is used that is provided with a dye layer wherefrom dyed portions or incorporated dyes are transferred onto a contacting receiver element by the application of heat in a pattern normally controlled by electronic information signals. Thermal dye transfer printing materials are described, for example, in EP-B 133 012 and EP-A 133 011.
Processes in which image formation is obtained by the image-wise transfer of an ingredient necessary for the chemical or physical process bringing about changes in colour or optical density to a receptor element are, for example, described in EP-A 671 283 in which a thermographic process is provided using
(i) a reductor donor element comprising on a support a donor layer containing a binder and a thermotransferable reducing agent capable of reducing a silver source to metallic silver upon heating PA1 and (ii) a receiving element comprising on a support a receiving layer comprising a silver source capable of being reduced by means of heat in the presence of a reducing agent, the thermographic process comprising the steps of PA1 i) said resistor arrays of adjacent rows of thermal heads are separated by a spacing, d, greater than 0.2 mm; and PA1 ii) said substrates of thermal heads in adjacent rows include an angle .THETA. comprised between arc sine (d/2D) and arc sine (2d/D).
bringing the donor layer of the reductor donor element into face to face relationship with the receiving layer of the receiving element, PA2 image-wise heating a thus obtained assemblage by means of a thermal head, thereby causing image-wise transfer of an amount of the thermotransferable reducing agent to the receiving element in accordance with the amount of heat supplied by the thermal head and PA2 separating the donor element from the receiving element.
This printing method is further referred to as `reducing agent transfer printing` or `RTP`. Materials for such processes are, for example, described in EP-A's 671 283, 671 284, 674 216, 677 775, 677 776, 678 775, 682 438, 683 428 and 706 080.
Direct thermal thermography is concerned with materials which are substantially not photosensitive, but are sensitive to heat or thermosensitive. Imagewise applied heat is sufficient to bring about a visible change in a thermosensitive imaging material. Most of the "direct" thermographic recording materials are of the chemical type. On heating to a certain conversion temperature, an irreversible chemical reaction takes place and a coloured image is produced. This irreversible reaction can be, for example, the reaction of a leucobase with an acid to produce the corresponding dye or the reduction of an organic or inorganic metal compound (e.g. silver, gold, copper or iron compounds) to its corresponding metal thereby producing a visible image. Such imaging materials are described, for example, in U.S. Pat. No. 3,080,254, EP-B 614 770, EP-B 614 769, EP-A 685 760, U.S. Pat. No. 5,527,757, EP-A 680 833, U.S. Pat. No. 5,536,696, EP-B 669 876, EP-A 692 391, U.S. Pat. No. 5,527,758, EP-A 692 733, U.S. Pat. No. 5,547,914, EP-A 730 196 and EP-A 704 318.
The ensuring of constant print quality with all these imaging materials over the whole material width when wide format printers are used places considerable demands upon the thermal head. Thermal heads with lengths greater than 30.5 cm (12 inches), typically 61.0 cm (24 inches) or 91.4 cm (36 inches), are difficult to produce with a single ceramic substrate. This is particularly the case for thin film thermal heads. It is therefore necessary to produce such long thermal heads by mutually joining, i.e. butting, a plurality of shorter thermal heads.
If these shorter thermal heads are placed in line the unprinted areas between the ends of butting thermal heads are so large that white-out (no image formation) is observed in the printed images. However, if these shorter heads are displaced laterally parallel to one another and the ends of the heads positioned so that no whiteout occurs, the degree of contact between the thermal head and the recording material varies along the length of the head leading to print density, image hue variations and even pinholes in the image.
Furthermore, in the case of thermographic recording materials on the basis of silver behenate with reducing agents such as 3,4-dihydroxybenzoic acid esters, as disclosed in EP-A 692 733, or gallic acid esters, which upon imagewise heating with a thermal head exhibit excellent image tone, non-uniform thermal contact during imagewise heating leads to pinholes in those parts of the resulting image corresponding to poorer thermal contact. These pinholes are probably due to carbon dioxide formation during the image-forming process, owing to the lower temperature attained by the thermographic recording materials under such conditions.
A solution for the problem of non-uniform thermal contact has been proposed in U.S. Pat. No. 5,367,321, which discloses a plurality of rows of heating resistance elements each of the rows being linearly disposed on an associated heat reserve layer wherein adjacent two of the heat reserve layers have cross-sections overlapping with each other, and adjacent two of the rows of the linear heating resistance elements on the two reserve layers are deviated from each other by a distance ranging from 0.2 to 1.5 mm in a subscanning direction perpendicular to the principal scanning direction.
This solution solves the problem when the thermographic recording material support is flat, but, for reasons of thermal development module compactness, a thermal development module is usually designed with a drum to support the thermographic recording material. When the thermographic recording material is drum-supported, this solution results in variation in print density, variation in image tone and even in pinholes in the image, which become worse when the diameter of the drum support is reduced and hence as the compactness of the thermal development module is increased.
There is therefore a need for an assembly comprising a plurality of thermal heads for wide format printing which provides uniform print density and uniform image hue without pinholes for use with a thermographic recording material supported on a drum, thereby enabling compact thermal development assemblies to be developed for wide format printers.